Together with findings of previous studies, which suggested that almost all KIT mutations were somatic rather than germline (reviewed in [3]), our present data support the idea that in familial cases (i.e., the majority of MCAD) mutated disease-related operator and/or regulator genes could be responsible for the development of somatic mutations in KIT and other proteins involved in the regulation of mast cell activity [3]. The gene discussed is KIT; the disease is medium chain acyl-CoA dehydrogenase deficiency.